1. Field of the Invention
The present invention relates to an image forming apparatus, and more particularly to a technology of controlling an electric power supply to a fixing device for fixing a toner on a recording sheet by applying a heat.
2. Description of the Background Art
There is known an image forming apparatus such as a copier, wherein a fixing roller is heated by applying a large electric current to a fixing heater with use of a commercial power source and an auxiliary power source device e.g. an electric dual-layer capacitor to reduce a time required for the fixing roller to reach a predetermined fixing temperature when the apparatus is in an initial startup condition or in an energy saving mode where the apparatus is in a standby state for a printing operation, with its power supply to the fixing heater being suspended. In the image forming apparatus, the voltage of the electric dual-layer capacitor itself to be used as the auxiliary power source device is relatively low e.g. 2.5V. In view of this, e.g. Japanese Unexamined Patent Publication No. 2003-297526 proposes use of a voltage boosting circuit. Specifically, in the publication, an electric power of the electric dual-layer capacitor is supplied to the fixing heater, after the voltage of the electric dual-layer capacitor is boosted to a voltage suitable for supplying to the fixing heater by the voltage boosting circuit.
Generally, a halogen heater to be used as the fixing heater has a low resistance in a normal temperature condition before being heated. When a voltage is applied to the halogen heater at the start time of heating the heater, an inrush current of a value several times or several ten times as high as that of a rated current flows to the heater. As a result, a coil in the voltage boosting circuit is saturated by the inrush current, which hinders the voltage boosting circuit from functioning properly. In view of this, the electric dual-layer capacitor and the voltage boosting circuit are controlled not to supply their electric powers to the heater at the start time of heating the fixing roller when an inrush current flows. If the aforementioned control of avoiding an inrush current is performed by causing the electric dual-layer capacitor and the voltage boosting circuit to wait for their electric power supply operations until the inrush current disappears, a startup time required for the fixing heater to perform an intended heating operation may be extended, which may resultantly increase a time required for the fixing roller to reach the predetermined fixing temperature. Also, there is proposed an idea of providing an inrush current preventing circuit between the voltage boosting circuit and the fixing heater to prevent an inrush current from flowing to the fixing heater so as to drive the voltage boosting circuit while suppressing flow of the inrush current. In this arrangement, the inrush current preventing circuit is an indispensable element.